The present invention relates to apparatus and method for protecting switching elements used in an inverter from being overheated applicable to a vector control system for an induction motor.
In a case where an induction motor is driven by means of the inverter, either one of upper or lower positioned switching element in one arm of the inverter is conducted (turned on). In a low frequency driving of the motor, the duration of time during which either of the upper or lower positioned switching element is relatively long, so that a temperature ripple in an internal junction of the one of the switching elements becomes large. In detail, as an output frequency of the inverter becomes low, the element internal junction temperature becomes higher if a total loss of the one of the switching elements remains unchanged.
FIGS. 5A and 5B show change patterns of the output current of the inverter and the temperature ripple of the switching element junction temperature during a normal driving (relatively high frequency driving) of the induction motor.
FIG. 5C and 5D show change patterns of the output current of the inverter and the temperature ripple of the switching element junction temperature during the low frequency driving of the same motor where an effective value of the output current is equal to the case of FIG. 5A.
As shown in FIGS. 5A to 5D, the junction temperature of the switching element becomes high during the on state of the switching element and becomes low during the off state thereof. That is to say, the temperature rise and drop are repeated for each of a half period of the output frequency. The difference between the maximum temperature and the minimum temperature is called the temperature ripple. In the case of the low frequency driving as shown in FIG. 5B, the duration of time during which the one of the switching elements is conducted (turned on) or turned off becomes long so that the temperature ripple becomes large.
Japanese Patent Application First Publication No. Heisei 7-255166 published on Oct. 3, 1995 exemplifies a method for protecting power switching elements used in an inverter.
In the disclosed power switching element protecting method, an output current of the inverter to a load (induction motor) is detected by means of a current detector to protect the switching elements from being overheated and a controller for the inverter is installed in which an on-and-off control for each of the switching elements is carried out so that the output current does not exceed a predetermined current value.
For example, a time limit characteristic of each switching element used for controlling the on-and-off of the switching elements is such that as time passes, the output current (load current) becomes lower. That is to say, each of the switching elements is protected from being overheated by providing a limit for the magnitude of the output current of the inverter and the duration of the output current flow.
However, since only the magnitude of the output current and a casing temperature of the one of the switching element are detected and read in the above-described Japanese Patent Application Publication, it is not sufficient for each of the switching elements to be protected from being overheated in such a low frequency, a large current driving situation as described above.